1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a fixing device for an image forming apparatus such as a copying machine or a printer, and an elastic roller to be employed in such image heating apparatus.
2. Description of Related Art
In the image forming apparatus such as the copying machine or the printer, it is recently desired strongly to reduce the electric power consumption and to shorten a time required for completing a first print (first print time).
In one of the means for achieving such objects, one of paired rotary bodies constituting a fixing nip portion of a heat fixing device is made thinner (made flexible) to reduce the heat capacity. For example there are proposed a type in which a flexible rotary body is sandwiched between an oblong plate-shaped heater and a pressure roller and a recording material is passed between the flexible rotary body and the pressure roller to thermally fix an image on the recording material, and an induction heating type in which heat is generated by the flexible rotary body itself by electromagnetic induction instead of employing the plate-shaped heater.
In the image heating apparatus utilizing such flexible rotary body, there is often employed an elastic roller-driven type configuration in which an elastic roller (for example pressure roller) is driven and a flexible rotary body is rotated by the rotation of the elastic roller, because there can be realized a simple configuration of rotating a pair of rotary bodies which are composed of a flexible rotary body and an elastic roller.
In an apparatus of such elastic roller-driven apparatus, for example an apparatus employing a fixing film as the flexible rotary body and a pressure as the pressure roller, in case the pressure nip width and the pressurizing force increase between the fixing film and the pressure roller, a sliding resistance increases between the fixing film and a sliding member which is provided inside the fixing film to form a nip portion in cooperation with the pressure roller, whereby a larger driving force is required for the pressure roller.
In case the driving force for the pressure roller is insufficient, the driving force of the pressure roller is not sufficiently transmitted to the fixing film when the recording material passes between the fixing film and the pressure roller, whereby a slippage is generated between the pressure roller and the recording material, resulting in an unsatisfactory conveying thereof and eventually leading to a jamming.
Also for preventing a fixing offset phenomenon, a parting layer is generally formed on the surface of the fixing film and the pressure roller, but the aforementioned slippage tends to be generated in case the surface layer of the pressure roller is formed with tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) excellent in durability and parting property.
Japanese Patent Application Laid-open No. 9-126225 (U.S. Pat. No. 5,722,026) provides a fixing apparatus in which the friction coefficient of the surface layer of the pressure roller is elevated by mixing a resin of a high friction coefficient in the fluorinated resin constituting the surface layer of the pressure or forming a coarse surface in the surface layer of a sheet non-passing area, thereby increasing the conveying force of the fixing film. In such method, however, a larger surface coarseness may result in a smear on the pressure roller or may reduce the strength of the surface layer, thereby leading to an insufficient durability. Also in case the surface layer is made coarse only in the sheet non-passing area only, the numerical range of the surface coarseness required for the practical use is too narrow and not practical.
It is an object of the present invention to provide an image forming apparatus to prevent a rotary body or a pressuring body from contaminating their surfaces for a long time.
Another object of the present invention is to provide an image forming apparatus for heating an image formed on a recording material, comprising:
a flexible rotary body;
a sliding member, provided in the flexible rotary body, for contacting the rotary body; and
an elastic roller for forming a nip portion for binding and conveying the recording material, in cooperation with the sliding member across the rotary body, the elastic roller being provided with an elastic layer and a parting surface layer,
wherein an area which is not passed by the recording material in the nip portion includes an exposed area where the elastic layer of the elastic roller is exposed,
and wherein the exposed area includes a portion where a diameter of the exposed area is equal to or less than a maximum diameter of an area of the parting surface layer.
Another object of the present invention is to provide an image heating apparatus for heating an image formed on a recording material, the apparatus including:
a flexible rotary body;
a sliding member, provided in the flexible rotary body, for contacting the flexible rotary body; and
an elastic roller for forming a nip portion for binding and conveying the recording material, in cooperation with the sliding member across the rotary body, the elastic roller being provided with an elastic layer and a parting surface layer;
wherein an area which is not passed by the recording material in the nip portion includes an exposed area where the elastic layer of the elastic roller is exposed,
and the exposed area includes a portion where a diameter gradually increases toward an end portion of the elastic roller.
Another object of the present invention is provide an elastic roller including:
an elastic layer; and
a parting surface layer;
wherein an end portion of the elastic roller in the axial direction includes an area where the elastic layer is exposed, and the exposed area includes a portion where the diameter gradually increases toward an end portion of the elastic roller.
Still other objects of the present invention will become fully apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings.